Ok, I just fixed it here so it wil be fixed in the next reease.
You can fix it immediately if you edit the stored procedure mp_Forums_RecalculatePostStats.
Look near the botton there is an update statement with:
MostRecentPostUserID = @MostRecentPostUserID,
change it to:
MostRecentPostUserID = COALESCE(@MostRecentPostUserID,0),
Thanks for the bug report.
Best,
Joe